I assume the ATX convertor is similar,if not the same, as that sold by Amigakit whereby the ATX lead from the power supply plugs onto the converter and the P8/P9 plugs from the Amiga complete the circuit. If the power switch is attached to the converter it should work - that was my set up before I purchased a mediator.
Might be worth testing with another power supply.
What a pity Power Computing's standards regarding the Amiga have stooped so low. I've ordered lots of stuff from them and have had no complaints regarding the service. Mind you my last order was a few years ago. However compared to Amigakit there is no comparison and their service is excellent.
Hope you get this sorted !!